Ngmoco co-founders Neil Young and Bob Stevenson have stepped down from the company, leaving their positions as CEO and CCO. Ngmoco parent company DeNA appointed former head of studios, Clive Downie as CEO to replace the outgoing Young.

According to today’s announcement, Young and Stevenson are moving on to “new adventures”, although Young will remain on DeNA’s board of directors.

DeNA acquired Ngmoco just over two years ago for $403 million — a deal that still stands as one of the biggest mobile gaming acquisitions. At the time, Ngmoco was one of the leaders in the mobile gaming space, having hit early success with titles like We Rule and Touch Pets. And while the purchase saw Ngmoco’s own game development efforts largely sidelined (the company has only released a handful of first-party titles since 2010), the past two years have seen the slow, but successful launch of DeNA’s Mobage gaming platform in the west.

“Over the last several years, I’ve had the opportunity to run many different parts of our organization, which has afforded me full visibility into our operations,” Downie said in a statement. “As we move into our next stage of growth, with our business starting to fire on all cylinders here in the West, I’m incredibly excited about our prospects and our mission to become the number one social mobile platform company in the world.”

DeNA now has 77 international titles live and has successfully transplanted the highly lucrative card battle genre to the West with Rage of Bahamut. The game is currently the No. 6 top grossing iPhone app and the No. 1 grossing Android app — a position its held for the past five months.  DeNA reported its gaming business generated $527.4 million in revenue last quarter with a net income of $127.1 million, with international titles like Ninja Royale and Blood Brothers seeing average revenue per daily active user (ARPDAU) in the range of $1.00. That said, DeNA’s successes are likely coming with a significant marketing outlay — DeNA spent $68.6 million just on sales, promotions and advertising in the same quarter.

Overall, 2012 seems to be a year of transition for DeNA. In February the company’s Gameview Studios subsidary lost co-founders Riz Virk and Mitch Liu after they hit their earnout dates and moved on to start new projects. After Ngmoco passed its own earnout date its vice president of social applications Jason Oberfest left to co-found a mobile health startup, followed shortly afterwards by Ngmoco’s chief publishing officer Simon Jeffery.